This is the mission where you destroy the yacht and then you have to get the arms dealers in the boats.

How the fuck do you aim while trying to press W and 8 at the same time? when I get in range the damn helicopter gets too low so I have to try to gain height by pressing W and then moves forward and down for a few seconds. If it is really low it just crashes into the water. Also the mini guns seem to about as much use as firing a pistol.

The closest I came to finishing the mission was when the two remaining boats stopped near the oil refinery. But after a few seconds of not being able to destroy them a message said the arms dealers got away.

This is the first mission that I can't complete - I've done all of the missions in GTA IV and TLAD.